Depression affects roughly 350 million people worldwide. It is the number one psychological disorder in the western world, yet the biological cause of depression remains unknown. Antidotal evidence indicates disruption of normal circadian rhythms is associated with increased risk of depression, such as shift workers and those suffering from seasonal affective disorder. Preliminary data indicate that chronic stress results in decreased expression of Per2, a circadian clock gene, in the bed nucleus of the stria terminalis (BNST), which is a brain area known to regulate emotional responses. The aim of this research project is to investigate the role Per2 plays in regulating depressive like behaviors. The experimental design includes manipulating Per2 expression by injecting short inhibitory (si)RNA or control siRNA into the bed nucleus of the stria terminalis and examining the onset of depressive-like behaviors. I expect that the injection of siRNA Per2 in the BNST will reduce Per2 expression and lead to depression in rats. This will be evident by increased floating in a forced swim test and decreased sucrose preference.